1. Field of the Invention
The present invention relates to an image stabilization device for a camera, wherein image displacement caused by a camera-shake due to hand trembling can be eliminated by driving an imaging system such as an optical system.
2. Related Background Art
Various conventional image stabilization devices have been proposed. In such a device, an image displacement on an imaging surface of e.g., a film due to camera-shake caused by a hand trembling or the like, is suppressed such that a lens system as an object to be controlled is driven in a vibration suppression direction.
For example, a camera vibration (normally, a camera vibration with respect to a photographing optical axis) is detected as an acceleration signal, and this acceleration signal is integrated by a signal processing system to obtain a displacement signal (or a velocity signal). The lens system is driven by these signals in a lens vibration suppression vibration of an image).
FIG. 18 is a diagram of a typical arrangement showing a principle of an image stabilization device including a conventional signal processing system of the type described above. An accelerometer (Rot Acc) 1 detects a camera (not shown) vibration with respect to a photographing optical axis as an acceleration signal. A detected acceleration signal a is integrated into a velocity signal v by a first integrator 2. The velocity signal v is then converted into a displacement signal d by a second integrator 3.
An actuator 5 is operated to drive a radially displaceable camera imaging system 4 (normally, an imaging lens system) in the radial direction to achieve image stabilization in accordance with the displacement signal d.
A variable resistor 6 constitutes a position detecting means for detecting an actual positional displacement of the imaging system 4. A signal from this position detecting means is fed back to an input system of the actuator, thereby constituting a feedback loop for matching radial position of the imaging system 4 with the vibration displacement.
A spring 8 urges the imaging system 4 toward a one-side limit position of its movable range during inactivation of the actuator 5. Unnecessary movement of the imaging system 4 during inactivation of the actuator 5 is thus prevented.
In the conventional arrangement described above, a radial position of the imaging system 4 upon activation of the actuator 5 is determined by a balance between the spring force of the spring 8 and a driving force generated by the actuator 5. In order to optimize an image stabilization start operation, an imaging system centering means as an initial position setting means is generally provided due to the presence of the spring 8.
The above operation will be briefly described. An overall radial stroke of the imaging system 4 in the above arrangement is defined as l, and an origin is defined as a central position (i.e., an /2 position) of the imaging system 4. Then, the imaging system 4 is urged at the -l/2 position by the spring 8 during inactivation of the actuator 5. When the actuator 5 is activated, the imaging system 4 must start an image stabilization operation while being kept urged at the -l/2 position if the centering means is not arranged. As the imaging system 4 is located at a negative limit position, it cannot be further moved in the negative direction. Therefore, a good image stabilization effect cannot be expected.
In order to arbitrarily move the imaging system in the positive or negative direction upon activation of the actuator 5, the imaging system centering means is added to immediately move the imaging system 4 from the -l/2 position to the origin at the activation start timing of the actuator 5 (This operation is called a centering operation). Image stabilization is started after the centering operation by the imaging system centering means is completed. A centering operation time is ideally almost zero. However, in practice, the centering time is about 30 to 100 msec due to an operating time of the imaging system 4 and a vibration damping time after centering.
The centering operation is utilized not only at the start of actuator operation but also during image stabilization control as needed. That is, the stroke of the actuator 5 and outputs from the integrators 2 and 3 are not infinite, and the imaging system may be moved to the stroke limit position within the camera (lens barrel) due to large vibrations. In this case, when the outputs from the integrators 2 and 3 are reset to re-start the centering operation of the imaging system, subsequent image stabilization control can be optimized.
In recent years, most of the commercially available cameras incorporate AF (Auto Focus) units for automatically focusing an image so as to reduce, for example, a load from a photographer. An application of the image stabilization device to an AF camera poses some problems. Prior to a description of these problems, an AF unit will be generally described.
Various types of AF unit are available. A single-lens reflex camera having many interchangeable lenses employs a TTL passive AF unit to cope with focal lengths of various interchangeable lenses from a wide angle lens to a telephoto lens. FIGS. 19(a) to 19(c) show operating states of such a TTL passive AF unit. This AF unit includes a field lens 11 located on an optically equivalent plane to a film surface as a primary imaging plane, a photographing lens 27, and secondary imaging lenses 13a and 13b. Two beams passing through different areas of the photographing lens 27 are independently sampled, and space images formed on the primary imaging plane are formed on distance measuring sensors 14a and 14b again. Each distance measuring sensor comprises a line photoelectric transducer element such as a BASIS or a CCD. Automatic gain control (AGC) for adjusting the photographing condition to the brightness of external light is generally performed by changing an accumulation time of the photoelectric transducer element.
In this AF unit, an in-focus state (FIGS. 19(a) and 20(a)), a forward focus state (FIGS. 19(b) and 20(b)), and a backward focus state (FIGS. 19(c) and 20(c)) are detected in accordance with distances between the object images on the distance measuring sensors 14a and 14b. A photographing lens drive mechanism (not shown) is driven in accordance with the detected state, and automatic focusing or focus adjustment can be achieved.
A camera with a telephoto lens is inevitably vibrated by the operator's hands or even if a tripod is used due to wind. This problem also occurs even in a camera having an AF unit. It is therefore also effective to mount an image stabilization device in the AF camera.
The following problem is posed when the image stabilization device and the AF unit as independent components are mounted in a camera.
Assume that the imaging system is moved to perform image stabilization in the radial direction while a distance measuring operation of the AF unit is being performed. In this case, displacement of an image formed on the distance measuring sensor can be prevented to obtain a good distance measuring effect. However, in the image stabilization device for centering the imaging system to the origin at the start of image stabilization operation, if the centering operation and the distance measuring operation are simultaneously performed, an error often occurs.
A cause of this erroneous operation will be described below.
Assume that charge is accumulated by the photoelectric transducer element serving as a distance measuring sensor, and that the imaging system centering operation of the image stabilization device is being performed. Under these conditions, an object image on the distance measuring sensor is abruptly moved during the accumulating operation. For this reason, a distance measuring disable state occurs due to movement of the image, thus causing a distance measuring error.
The above problems are also presented in association with another device for detecting photographic information by using a photoelectric transducer means.
In addition, the centering operation poses a problem in association with an exposure operation of a silver chloride film or the like. That is, an image stabilization operation must be effective during film exposure. However, when the imaging system 4 is deviated from the center of the stroke and then a release operation is started, the imaging system 4 tends to abut against the stroke end on the side having a small stroke margin. Then, the image stabilization operation tends to be invalidated. For this reason, it is preferable that every time the release operation is started, the centering operation is performed to locate the imaging system 4 at the center of the stroke, and the release operation is started.
However, the centering operation requires a period of 30 to 100 msec. If the release operation is started during the centering operation, the imaging system 4 is moved independently of hand trembling while the shutter is open and the film is exposed to light. Therefore, an image which is displaced in the direction of movement of the imaging system 4 is recorded on the film surface.